1. Field of the Invention
The present invention relates to a process for producing crystalline ceric oxide particles. The ceric oxide is used as an abrasive, ultraviolet absorbing material, catalyst material, fuel cell material and the like. Out of these, the crystalline ceric oxide of the present invention provides an excellent material for use as an abrasive and an ultraviolet absorbing material.
Further, the present invention relates to the modification of the surface of a ceric oxide particle or a particle essentially composed of ceric oxide obtained by calcining and grinding or a composition containing a rare earth element essentially composed of cerium, as well as to an abrasive comprising the surface modified particles and a polishing method.
2. Description of the Related Art
Japanese Laid-open Patent Application No. Sho 63-502656 discloses a process for producing ceric oxide particles having a particle diameter of 0.05 to 10 .mu.m by holding an aqueous solution of cerium (III) nitrate or cerium (IV) nitrate in a sealed container at a temperature of 200 to 600.degree. C. and at a pressure of not less than 40 atm.
Japanese Laid-open Patent Application No. Hei 6-2582 discloses a process for producing crystalline ceric oxide particles having a particle diameter of not more than 300 angstroms which comprises cleaning a gel obtained by reacting a cerium salt compound and an alkali metal hydroxide or ammonia to remove impurities, adding an acid such as nitric acid or acetic acid to the gel, and subjecting the resulting mixture to a hydrothermal treatment at 100.degree. C. or more.
Japanese Laid-open Patent Application No. Hei 8-81218 discloses a process for producing ceric oxide particles having a particle diameter of 0.03 to 5 .mu.m which comprises adjusting pH values of a solution comprising ceric hydroxide and a nitrate to 8 to 11 using an alkaline substance and heating the solution at a temperature of 100 to 200.degree. C. under pressurization.
It is known that cerium is an element which is easily oxidized from valence (III) to valence (IV) in lanthanoids. For example, it is described on page 348 of "Inorganic Chemistry Vol. 1" written by Thunderson and published by Hirokawa Shoten on Apr. 25, 1982 that cerium (IV) is produced when an alkaline suspension of cerium (III) hydroxide is exposed to air.
Since a hydrothermal treatment is carried out at a temperature of not lower than 100.degree. C. under such a condition that a corrosive substance such as nitric acid or acetic acid is contained in all the processes of Japanese Laid-open Patent Application Nos. Sho 63-502656, Hei 6-2582 and Hei 8-81218, a high-pressure container that meets the reaction conditions is required and further acid-resistant Teflon, glass or a corrosion-resistant alloy such as hastelloy must be used as a material for the high-pressure container.
By the way, it has been proved that cerium oxide particles or the particles of a composition essentially composed of cerium oxide have excellent performance as an abrasive for polishing inorganic glass, quartz crystal and quartz glass.
Japanese Laid-open Patent Application No. Sho 58-55334 discloses a process for producing a dispersible product containing a cerium compound which comprises heating a cerium (IV) oxide hydrate in the presence of a salt and disintegrating agglomerated fine crystals contained in the cerium (IV) oxide hydrate. The above publication teaches that ammonium nitrate is used as the salt in addition to a metal nitrate, metal chloride and metal perchlorate. The publication further teaches that a solution containing a cerium (IV) oxide hydrate and ammonium nitrate was dried at 105.degree. C. and further heated at 300.degree. C. to obtain a product containing ceric oxide and a nitrate which was then dispersed in water to obtain a gel, as an embodiment of the invention. However, the description is not made about the application of this gel.
Japanese Laid-open Patent Application No. Hei 5-262519 discloses a process for producing a rare earth oxide which comprises mixing an oxalate, rare earth compound and ammonium salt in an aqueous medium, separating a precipitate produced at 30 to 90.degree. C. and calcining the obtained oxalic acid rare earth ammonium double salt at 600 to 1,200.degree. C. The above publication teaches that ammonium nitrate, ammonium chloride, ammonium acetate or the like is used as the ammonium salt and cerium nitrate is used as the rare earth compound.